When a Satellite Positioning System (SATPS) receiver/processor powers up, or when the receiver/processor experiences SATPS signal interruption, if the receiver/processor has no almanac that indicates the present location of the visible SATPS satellites, the receiver/processor and associated SATPS antenna will perform a blind satellite search to find a sufficient number of SATPS satellites, usually three or more, to begin establishing the antenna's SATPS determined location and/or proper time. The SATPS antenna and receiver/processor will usually select SATPS satellite numbers at random for the search. This procedure will often consume several minutes before "lock" on an adequate number of SATPS satellite signals is achieved. Several workers in electrical communications have disclosed methods and/or apparati for reducing the time or difficulty of acquiring signals communicated from satellites.
U.S. Pat. No. 4,384,293, issued to Deem et al, discloses apparatus for providing pointing information, using one or more GPS satellites and two antennas spaced apart about ten carder signal wavelengths. The difference in phase of GPS signals received by the two antennas determines the pointing direction determined by the line of sight between the two antennas. Phase differences of GPS signals received by arrays of three or more collinear or non-collinear antennas are used to determine the attitude of an object on which the antennas are mounted in U.S. Pat. No. 5,021,792, issued to Hwang, and in U.S. Pat. No. 5,101,356, issued to Timothy et al.
Sekine discloses GPS receiver/processor apparatus that quickly maximizes correlation between a received GPS pseudo-random noise (PRN) code and an internally stored GPS code, in U.S. Pat. No. 4,968,981. This approach uses a separate channel for each of N PRN codes and shifts the phase of the internally stored code n/2 bits at a time (n=1, 2, . . . , N), in a search for a position of increased code correlation value.
In U.S. Pat. No. 5,036,329, Ando discloses a satellite reacquisition or initial acquisition method applicable to GPS satellites. Using an estimate of the average Doppler shifted frequency f.sub.avg manifested by the GPS signals received from a visible GPS satellite, a narrow band search is first performed in the frequency range f.sub.avg -8600 Hz.ltoreq.f.ltoreq.f.sub.avg +8600 Hz. If no GPS satellite signals are found in this range within 3.75 minutes, the search range is widened until at least one GPS satellite signal is found.
A simultaneous multi-channel search for reacquisition of GPS satellite signals after signal interruption occurs is disclosed by Sakaguchi and Ando in U.S. Pat. No. 5,059,969. This method first searches for the GPS satellite with the highest elevation angle relative to the GPS antenna. Two or more sequences of signal frequency ranges are swept over in parallel until at least one GPS signal is reacquired.
U.S. Pat. No. 5,061,936, issued to Suzuki, discloses attitude control for a rotationally mobile antenna. If the strength of the initial signal received by the antenna from a spacecraft (whose position is yet unknown) is below a first selected threshold and above a second selected threshold, the antenna attitude is scanned over a relatively small range, to increase the signal strength toward or above the first threshold value. If the signal strength is initially below the second threshold, the antenna attitude is scanned over a larger range, to increase the signal strength above the second threshold value so that a smaller range antenna scan can be implemented.
In U.S. Pat. No. 5,119,504, Durboraw discloses a satellite-aided cellular communications system in which a subscriber unit self-determines its own (changing) location and transmits this information to the satellites for use in subsequent communications. This requires that each subscriber unit transmit and receive signals, and one subscriber unit does not communicate directly with, or provide satellite location information for, another subscriber unit.
An electronic direction finder that avoids reliance on sensing of terrestrial magnetic fields for establishing a preferred direction for satellite signal acquisition is disclosed by Ghaem et al in U.S. Pat. No. 5,146,231. The apparatus uses a receiver/processor for GPS or similar navigation signals received from a satellite, and requires (stored) knowledge of the present location of at least one reference satellite from which signals are received. The orientation of the finder or its housing relative to a line of sight vector from the finder to this reference satellite is determined. This orientation is visually displayed as a projection on a horizontal plane. Any other direction in this horizontal plane can then be determined with reference to this projection from a knowledge of the reference satellite location.
Ando, in U.S. Pat. No. 5,155,491, discloses a method for tracking radio signals from GPS satellites that follow a single orbit around the Earth. At most four GPS satellites follow one of the six GPS orbits, as the constellation is presently configured. The C/A-code and/or P-code is known for each of the at-most-four GPS satellites in a single orbit so that searching along a single orbit requires acquisition of only one of the four known codes associated with these satellites, and at least one of these four GPS satellites is not visible at a particular observation time. After acquisition of whatever GPS satellites on a particular GPS orbit can be tracked, the system moves sequentially from one GPS orbit to another orbit until all trackable GPS satellites are found. The system then selects the three or four GPS satellites that are most suitable for global positioning computations.
These methods either require storage of detailed knowledge of the satellite trajectories or of satellite signal indicia. This information for SATPS satellites can be voluminous and is not present in many SATPS signal receiver/processor systems. What is needed is a method that relies only upon information that is already available within the receiving system or from another nearby receiving system. Preferably, the method should provide reasonably accurate information on the present location of any visible SATPS satellite, should allow rapid acquisition of SATPS signals from one or a plurality of visible SATPS satellites, and should not require consumption of much additional power for operation.